Many buildings employ an HVAC system to maintain a comfortable environment. The HVAC system provides heating and cooling for the buildings. Conventionally, local engineering knowledge is used to produce a sequencing that arranges equipment (chillers, pumps, etc.) by their energy efficiency such as using the least efficient equipment only under the most extreme circumstances. To achieve a better energy efficiency and energy savings for the HVAC system, it is typically advantageous to use the equipment with the higher efficiency before bringing on other equipment that uses more energy to do the same job.
HVAC systems are designed with multiple pieces of equipment, some to do the same job as other identical pieces of equipment, like chillers in which there may be two or more water chillers as part of the building or campus HVAC system. During low cooling season only one of the chillers would be operating; whereas during high cooling season maybe all of these chillers would be operating. In addition, one or more chillers may be purposefully kept off line for a variety of reasons such as, but not limited, repairs, maintenance, etc.
Even though these chillers may be similar or identical (e.g., same manufacture, same model, etc.), the chillers may often have different efficiency's for performing the same heating or cooling task. By way of example, a chiller efficiency is typically measured as kW/ton (or other HVAC equipment efficiency measurements like Co-efficient of Performance (COP), Energy Efficiency Ratio (EER), Seasonal Energy Efficiency Ration (SEER)), which is the amount of energy (measured in kW) used by the chiller to produce cooling (measured in tons). A lower kW/ton rating indicates higher efficiency (tons=one ton of cooling is the amount of heat absorbed by one ton of ice melting in one day, which is equivalent to 12,000 Btus per hour, or 3.516 kilowatts (kW) (thermal energy)).
A number of different methods have been developed to measure equipment efficiency and stage equipment by their efficiency ratings. These methods include observation and manual modeling of equipment efficiency for certain building conditions (usually including wet bulb air temperature, building load, etc.) along with use of a manufacturer's specification for equipment efficiencies. These parameters are combined with engineering knowledge to provide a static sequencing order that may be used over a period of time to keep energy efficiency high.
While equipment like a chiller is operating, the efficiency of each operating chiller may be measured and compared by an engineer, a building automation system (BAS) or an energy management system (EMS) with the correct instrumentation. By way of example, some engineers may specify the most efficient sequence to run the equipment and that sequence may be controlled manually by operators or it may be hard coded within the BAS to run in that particular sequence in an automated manner.
Many of the conventional sequencing methods involve many hours of labor on the part of a knowledgeable engineer and the sequence modeling typically occurs only at the outset when the equipment is initially commissioned. The “stage and forget” process may be problematic since equipment energy efficiencies can drift over time as parts or components wear, critical operating fluids leak or are used up, and/or conditions change in the system as a whole. Such operational changes directly affect the energy usage of individual pieces of equipment.
The reasons that equipment like chillers, pumps, fans, cooling towers or boilers may have different efficiencies may be due to (1) manufacturing differences (large or small); (2) poor equipment (3) equipment wear or broken parts; (4) contamination or loss of refrigerant within a chiller; and/or (5) fouling or buildup of material on working surfaces.
FIG. 1 shows a chart 10 in which energy efficiency 12 is recorded over time 14 for two separate, but otherwise identical chillers 16, 18, respectively. The upper curve 20 shows how the efficiency of first chiller 16 varies over time while the lower curve 22 shows how the efficiency of second chiller 18 varies over time.
Additionally, while the manual sequencing process may capture trends in equipment efficiency it is often unable to observe small fluctuations in efficiency. Since manufacturer specifications are often used in the manual sequencing process it can also be difficult to perceive differences in equipment energy efficiencies for the same model. This may result in operating less efficient equipment, using excess energy that may be “left on the table.”
The most predominant and common method to operate and stage equipment involves operating the equipment in an equal runtime rotation scheme. This method rotates the operating sequence of a group of equipment based on the accumulated running hours (or days, or minutes) of each piece of equipment. When one piece of equipment has accumulated a certain number of operational hours than another piece of equipment the operating sequence is rotated. The equipment having the lowest logged hours is rotated in the operating sequence to turn on first, while the equipment having the highest logged hours will be turned on last in the sequence.
Another method to operate and stage equipment involves using a minimum runtime sequence in which the equipment is staged to ensure that each piece of equipment runs for a certain amount of hours every rotation period, which is commonly done to make sure that the equipment does not sit inoperable for a long period. When equipment is left inoperable for long periods of time, the equipment may decay or lose critical operating fluids to leakage or evaporation.
FIG. 2 shows another chart 24 in measured energy efficiencies 26 are plotted over time 28. Each curve 30, 32, 34 represents the energy efficiencies 26 of three similar chillers respectively, over time. An upward facing arrow 36 indicates that chiller 30 had the best operating efficiency for a period of time, but then began operating at a poorer operating efficiency over a later period of time as indicated by arrow 38.
Over time, the energy efficiencies of the equipment may drift as shown in FIG. 2, and such drifts may be result in a reduced or poorer energy efficiency for that particular equipment. On the flip side, a piece of equipment with a poor energy efficiency may be improved through maintenance, repair, or cleaning, for example.